A representation of $\text{Out}\left(F_n\right)$ by counting subwords of cyclic words

Abstract

We generalize the combinatorial approaches of Rapaport and Higgins--Lyndon to the Whitehead algorithm. We show that for every automorphism $\phi$ of a free group $F$ and every word $u\in F$ there exists a finite multiset of words $S_{u,\phi }$ satisfying the following property: For every cyclic word $w$, the number of times $u$ appears as a subword of $\phi \left(w\right)$ depends only on the appearances of words in $S_{u,\phi }$ as subwords of $w$. We use this fact to construct a faithful representation of $\text{Out}\left(F_n\right)$ on an inverse limit of $\mathbb{Z}$-modules, so that each automorphism is represented by sequence of finite rectangular matrices, which can be seen as successively better approximations of the automorphism.